Thus, a small change in pupil size can cause a significant change in refraction. Doubling pupil diameter increases spherical aberration 16 times. The effect of spherical aberration increases as the fourth power of the pupil diameter. This is because most wave aberration is due to 2nd order aberrations, which have a square radius dependency. In general, the increase in overall wave aberration with pupil size has been reported to increase to approximately the second power of the pupil radius. As the pupil enlarges, more peripheral rays enter the eye and the focus shifts anteriorly, making the patient slightly more myopic in low-light conditions. In brighter conditions, the pupil constricts, blocking the more peripheral rays and minimizing the effect of spherical aberration. Spherical aberration exacerbates myopia in low light (night myopia). Spherical aberration is the cause of night myopia and is commonly increased after myopic LASIK and surface ablation. There are numerous higher-order aberrations, of which only spherical aberration, coma and trefoil are of clinical interest. In spherical aberration (Bottom) peripheral rays are focused more tightly than central rays. A perfect lens (top) focuses all incoming rays to a point on the Optical axis. Low order aberrations account for approximately 90% of the overall wave aberration in the eye. Other lower-order aberrations are non- visually significant aberrations known as first order aberrations, such as prisms and zero-order aberrations (piston). Low order aberrations include Myopia (positive defocus), hyperopia (negative defocus), and regular astigmatism. Most eyes show positive spherical aberration when unaccommodated with a trend toward negative spherical aberration on accommodation. The accommodative response of the eye results in changes to the lens shape and substantially affects the wavefront aberration pattern. The balance of corneal and internal aberrations is a typical example of creating two coupling optical systems. Besides, there is strong evidence of compensation for aberrations between the cornea and intraocular optics in cases of astigmatism (horizontal/vertical) and horizontal coma. The spherical aberration of the cornea is usually positive whereas the young crystalline lens exhibits a negative spherical aberration. Several studies have reported a compensation of the aberration of the cornea by the aberration of the crystalline lens. High order aberrations increase with age and mirror symmetry exists between the right and the left eyes. Higher order aberrations are a relatively small component, comprising about 10% of the eye's total aberrations. In normal population the dominant aberrations are the ordinary second-order spherocylindrical focus errors, which are called refractive errors. Therefore, aberrations of the eye are the difference between two surfaces: the ideal and the actual wavefront. The distance in micrometers between the actual wavefront and the ideal wavefront is the wavefront aberration, which is the standard method of showing the aberrations of the eye. Wavefronts are always normal (perpendicular) to the rays.įor light to converge to a perfect point, the wavefront emerging from the optical system must be a perfect sphere centered on the image point. Rays and wavefronts are two mutually complementary approaches to light propagation. The flat wavefronts change to spherical wavefronts as they pass through a pinholeĪ wavefront is a surface over which an optical disturbance has a constant phase. Wavefront approach to aberrations of the eye These adverse effects are particularly noticeable when the pupil is large. Several mechanisms may explain the increase in the amount of higher-order aberrations with conventional eximer laser refractive procedures: a change in corneal shape toward oblateness or prolateness (after myopic and hyperopic ablations respectively), insufficient optical zone size and imperfect centration. The appearance of visual complaints such as halos, glare and monocular diplopia after corneal refractive surgery has long been correlated with the induction of optical aberrations. It has only recently become possible to measure the aberrations of the eye and with the advent of refractive surgery it might be possible to correct certain types of irregular astigmatism. Correction of spherocylindrical refractive errors has been possible for nearly two centuries following Airy's development of methods to measure and correct ocular astigmatism. The optical quality of the eye is limited by optical aberrations, diffraction and scatter. The eye, like any other optical system, suffers from a number of specific optical aberrations.
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